In a comment in "Treating irregularity", Marcos Dumay de Medeiros says this about direct-carbon fuel cells:

It annoys me a lot your insistence in making your calculations with the carbon fuel cell. It is experimental!

All right, for the sake of argument let us assume that the direct-carbon fuel cell scheme has some show-stopping problem and it's not usable. Not for vehicular power, not in stationary applications, not anywhere. If it doesn't work, what are the options?

Humor aside, when reality creeps in I am not one to bust it for trespassing. I always have a plan B, and in this case plan B is...

zinc-air fuel cells! (Tell me you didn't know that was coming. I won't believe you, but it'll be good for a laugh.)

From a mole of carbon (93960 cal/mol), a mole of ZnO and an indeterminate amount of
heat, we get a mole of zinc metal (84670 cal/mol) and a mole of carbon monoxide
(68560 cal/mol) plus waste heat. Ignoring the waste heat, the 93960 cal of reactants
yields 153230 calories of products. The question becomes, can these make as much
useful output as a DCFC can make of raw carbon?

I believe so. Zinc metal is convertible to ZnO and electricity with an efficiency
of roughly 62%, and CO can be fed to either a molten-carbonate fuel cell or a solid-oxide
fuel cell; both can make electricity at an efficiency of roughly 60%. Here's what
we'd get out of a mole of carbon via the two options:

Table 2: Yield comparison

Reactant

ΔH, gram calories/mol

Converter

Efficiency

Yield, cal/mol

C

93960

DCFC

80%

75168

Zn

84670

Zn/air fuel cell

62%

52495

CO

68560

SOFC or MCFC

60%

41136

TOTAL

153230

93631

As long as you have a source of heat to drive the zinc reduction, you can get about
24% more total output using the zinc cycle compared to the direct-carbon system. There's
a second fallback too: if
neither the MCFC nor the SOFC are ready for widespread commercial use in time, carbon
monoxide makes a perfectly good gas-turbine fuel. It can probably be converted to work
as efficiently as natural gas, or about 55% in a combined-cycle plant. There's plan C.

Going back to dealing with irregularity, a carbon/zinc cycle helps in this way:

It adds another storable fuel, carbon monoxide, to the chain. CO can be stored
in spent gas wells and other gas-tight reservoirs.

It adds flexibility.

A direct-carbon fuel cell yields carbon dioxide,
which is mainly suitable for sequestration. The zinc reduction produces carbon monoxide,
which is a chemical feedstock as well as an energy source.

A system which depends on carbon as a feedstock halts when it runs out of fixed carbon.
Zinc metal can be produced from oxide either chemically (reduced with carbon) or elecrolytically;
this allows wind, solar, nuclear or hydro to substitute for carbon.

There's one more issue to deal with, and that's the dependence of the solar-thermal zinc
reduction system (ZnO + C + Δ -> Zn + CO) on cloudless days. There just aren't
many of those in some parts of the country that need energy. This is not a killer,
because solar heat is just the sexiest source of energy to drive the reaction; it could just
as easily be driven by surplus wind electricity (turning the immediate supply of wind power
into two different storable fuels) or by combustion of part of the carbon (sacrificing the
carbon monoxide byproduct). Either way, there's a reasonable alternative.

Marcos said: "But the problem is that we can't be sure about how long we'll need to wait until they can be used."

Nissan provided their assessment of future engine technologies in the December issue of Automotive Engineering International magazine. I suppose this is based on their own R&D/marketing projections. Anyway:

Personally I think this breakdown is crap but the notable figure for me is that even 40 years into the future, a major automotive manufacturer predicts a mere 16% market penetration for FCVs (and you have to assume they are talking hydrogen here).

E-P said: "As long as you have a source of heat to drive the zinc reduction, you can get about 24% more total output using the zinc cycle compared to the direct-carbon system."

Don't we have to consider the cost/efficiency of the source of heat in the equation? The advantage to the carbon cycle is that the heat required to generate carbon is minimal compared to that of the zinc cycle.

In the presence of carbon, solar energy can be stored in zinc. The source of this solar energy is a solar thermal device, probably a parabolic mirror, needed to generate the high temperature. What is the efficiency of this piece of the process? I would assume it's higher than that of a a parabolic mirror linked to a Stirling engine.

I'm sure this has occured to everyone, but in the worst case scenerio, the leftover carbon could be simply burned, at a much lower efficiency of course.

Another option would be to bury a portion of this carbon, if a main goal is to remove C from the atmosphere. The charcoal powder is probably easier to sequester than the C from the pyrolysis gases, though this could also be done.

Jonathan: No, no boron. That's George's hobby-horse; my calculations indicate that it's far more difficult than he thinks and may not work at all.

hamerhokie: You're right about the heat, but given the very different cost of heat from various sources and even the same source at different sites (solar), a calculation to that level of detail is way beyond what I could do for a quickie. I'm not sure I'd be able to find free data for that, period.

BBM: Carbon-negativity is definitely high on my list of desirable characteristics for a new energy system.

Boron may be George's hobby-horse; it certainly is mine too, plus there has been some recent ORNL/Dave Beach talk, although their approach seems to me to divide the leap over the dragon-filled crevasse into easy stages.